1. Field of the Invention
The present invention relates to an MSn type mass spectrometry apparatus for selecting an ion having a specific mass as a precursor ion, inducing fragmentation of the ion, and mass-analyzing the resulting product ions, and more specifically to an MSn mass spectrometry apparatus having a function of automatically selecting a precursor ion based on an MSn−1 spectrum acquired through an MSn−1 analysis to perform an MSn analysis.
2. Description of the Related Art
In MSn type mass spectrometry apparatuses, there has been known one type having a function of sorting a peak meeting a condition which is input and set by a user in advance, from peaks appearing on a mass spectrum acquired through an MS1 analysis, and automatically selecting an ion corresponding to the sorted peak as a precursor ion to perform an MS2 analysis. For example, a liquid chromatography/mass spectrometry system (LCMS-IT-TOF produced by Shimadzu Corp.) as disclosed in the following Non-Patent Document 1 comprises an ion trap (IT) for temporarily holding ions to sort the ions according to mass and then inducing fragmentation, and a time-of-flight mass spectrometer (TOF-MS) for mass-analyzing ions expelled from the ion trap with high mass resolution and accuracy. The liquid chromatography/mass spectrometry system has an automatic MSn function of automatically sorting a peak meeting a given condition based on a mass spectrum acquired through an MS1 analysis during an LCMS analysis, selecting an ion corresponding to the sorted peak as a precursor ion, inducing fragmentation of the precursor ion by the ion trap, and introducing various types of ions produced by the fragmentation into the TOF-MS to perform a mass analysis (MS2 analysis).
Generally, a large number of unwanted noise peaks having low peak intensities, such as peaks derived from foreign substances, appear on a mass spectrum. Thus, in order to prevent erroneous selection of such peaks, the automatic selection of a precursor ion is performed after a processing of excluding any peak having a peak intensity less than a preset lower limit value of a signal intensity on an MSn−1 spectrum. As the most common methodology for selecting a precursor ion, a technique of sorting peaks each having a peak intensity equal to or greater than a lower limit value in the above manner, and selecting ions corresponding to the sorted peaks as precursor ions in descending order of peak intensity is widely used (see the following Non-Patent Document 2).
However, the MS2 analysis using the above technique has the following problem. In a liquid chromatography/mass spectrometry system, a liquid sample containing sample components temporally separated through a liquid chromatography column is introduced into an ionization section of a mass spectrometry apparatus to perform mass analysis, and thereby peaks derived from a plurality of sample components exhibiting contiguous elution times (contiguous retention times in the column) appear on one mass spectrum, i.e., on a mass spectrum acquired at a certain time point. Thus, as the number of sample components exhibiting contiguous elution times becomes larger, a larger number of peaks derived from the different sample components appear on one mass spectrum, and therefore the number of types (a mass range) of ions to be selected as precursor ions is increased (widened). In this situation, if an MS2 analysis is performed while selecting such ions as precursor ions in descending order of peak intensity, it requires time before an ion exhibiting a relatively low peak intensity is subjected to the MS2 analysis.
Particularly, in cases where a large number (i.e., 10 cycles) of MS2 analyses are repeated for the same precursor ion to increase an S/N ratio of a mass spectrum, and acquired mass profiles are subjected to an integration processing to create an MS2 spectrum, a time period required for the analysis per precursor ion becomes longer. Thus, it requires further time before an ion exhibiting a relatively low peak intensity is subjected as a precursor ion to the MS2 analysis. Consequently, at a time when the MS2 analysis becomes able to be performed for the ion exhibiting a relatively low peak intensity as a precursor ion, elution of the corresponding component from the column is likely to already be completed to preclude acquisition of a sufficient (credible) MS2 spectrum for the component.
[Non-Patent Document 1] “Liquid Chromatograph Mass Spectrometer Systems (LCMS-IT-TOF)”, [online], Shimadzu Corp., [retrieval date: May 17, 2007], Internet <URL: http://www.an.shimadzu.co.jp/products/lcms/it-tof.htm>
[Non-Patent Document 2] Iida, et al., “Application of LCMS-IT-TOF to Proteome Analysis”, Shimadzu Review, Vol. 63 [1•2], pp. 19 to 28, Shimadzu Review Editorial Board, Sep. 29, 2006